Polarizers are important components of liquid crystal displays. Liquid crystal displays (LCDs) are widely used components in applications such as, for example, Notebook Personal Computers (PCs), calculators, watches, liquid crystal color TVs, word processors, automotive instrument panels, anti-glare glasses and the like. A useful review article, for example, is "Digital Displays" by in Kirk-Othmer Encyclopedia of Chemical Technology, Third edition, Volume 7, page 726 (1979), Wiley-lnterscience Publication, John Wiley & Sons, New York. Typically, Polarizers are used in the form of film, the polarizer film (also called polarizing film). In an LCD, the liquid crystal elements are generally sandwiched between two layers of polarizing films.
Traditional polarizing films comprise a stretched polymer film such as, for example, polyvinyl alcohol (PVA), a dichroic absorber and other optional layers. The dichroic absorber is usually iodine or a dichroic dye that is absorbed in the polymer film. However, there are several disadvantages with such films that make them unsuitable for advanced and sophisticated applications. Some such disadvantages include, for example, non-uniformity, separation of the absorber over time, susceptibility to moisture and the like. For this reason, liquid crystalline polymer-based polarizers are being developed for polarizers. The process of molding or extrusion generally achieves a high degree of stable orientation in such polymers.
Pending U.S. Pat. application, Ser. No., 08/460,288, filed Jun. 2, 1995, now U.S. Pat. No. 5,672,296 discloses novel liquid crystalline polymer compositions useful in polarizer applications. Illustrative compositions disclosed therein are liquid crystalline polyesters which comprise repeat units corresponding to the formula: EQU -P.sup.1 !m-P.sup.2 !n-P.sup.3 !q-
wherein P.sup.1, P.sup.2 and P.sup.3 represent monomeric moieties with P.sup.1 being an aromatic hydroxy carboxylic acid, P.sup.2 being an aromatic dicarboxylic acid and P.sup.3 being a phenol; and m, n and q represent mole percent of the respective monomers ranging from 5-70 mole percent individually. Additional monomers may also be present. A preferred composition in the same patent is a film-forming wholly aromatic thermotropic liquid crystal polyester which comprises five monomeric moieties derived from 4-hydroxybenzoic acid, terephthalic acid, 4,4'-dihydroxybiphenyl, 6-hydroxy-2-naphthoic acid, and resorcinol in a molar ratio 30:20:10:30:10 respectively. Such LCPs are converted to polarizing films by combining them with suitable dichroic absorbers and then melt extrusion to yield the films. After melt extrusion, the polarizer films are subjected to further processing steps which include lamination to an optically transparent polymeric layer which is generally a thermoplastic. This lamination may require the use of adhesives depending on the adhesion of the polymeric layer to the melt extruded polarizer film.
Melt extruded LCP films, however, typically exhibit a degree of machine direction-oriented surface texture and therefore need polishing prior to further use. In the case of LCPs for polarizers, the films, especially in the required thickness of 1 mil (25 .mu.m) or less, easily fibrillate and are also damaged easily during processes such as, for example, slitting and winding operations. Subsequent lamination of the polarizer films to the optically transparent polymeric layer preserves these defects and hence substantially reduces the utility of the film in devices where surface quality is critical for optical applications. Furthermore, preparing a polarizing film followed by further lamination to a thermoplastic involves extra steps which make the process uneconomical in addition to possible creation of additional defects.
One way to reduce the defects would be to coextrude the polarizer film with the optically transparent cover layer. This would avoid the separate lamination step later as well as the creation of additional defects on the polarizing LCP film. Coextruded laminates of LCPs containing other LCPs as surface layers on both sides are known. For example, U.S. Pat. No. 5,248,530 discloses such laminates prepared by coextrusion. Copending U.S. Pat. application, Ser. No. 08/761,042, now U.S. Pat. No. 5,744,204 filed of even date herewith, describes a coextrusion process for LCPs. In that case, the cover layers are non-adherent, and are peeled off after the coextrusion. A typical LCP for polarizer applications, on the other hand, is a mixture of an LCP polymer and a dichroic dye, where the dye has been dissolved in, blended with or absorbed in the LCP. A suitable process that provides a laminate from which the cover layers need not be peeled off will be highly useful for such LCPs.
There is a need for an improved polarizer laminate with improved surface characteristics.
It is an objective of this invention to provide improved quality LCP-polarizer laminates.
It is an additional objective of this invention to provide polarizing films with improved surface quality.
It is a further objective of this invention to provide LCP films which contain surface protecting films thereon that need not be delaminated during further processing steps.
It is a further objective of this invention to provide polarizer films which contain surface protecting films thereon that need not be delaminated but may be suitable for processing to a device by conventional processing methods.
It is a still further objective of this invention to provide polarizer laminates prepared by fewer number of steps and with fewer defects.
Other objectives and advantages of the present invention will be apparent to those skilled in the art as well as from the following description and Examples.